A programmable logic controller (PLC) is a digital computer typically configured to automate mechatronic processes, such as control of machinery on factory assembly lines, control of amusement rides, or control of lighting fixtures, etc. Those skilled in the art and others will recognize that PLCs are used in many different industries and machines such as packaging and semiconductor machines. Unlike general-purpose computers, a PLC is designed for multiple inputs and output arrangements, extended temperature ranges, immunity to electrical noise, and resistance to vibration and impact. Programs to control machine operation are typically stored in battery-backed or non-volatile memory. A PLC is an example of a real time system since output results are produced in response to input conditions within a bounded time, otherwise unintended operation will result.
An exemplary system environment 10 that employs FSK modems connected to PLCs is illustrated in FIG. 1. A system server 20, such as a desktop computer, is connected to a plurality of PLCs 30 through a network 40. The network 40 may be any type of known network, such as the Internet, LAN, WAN, or other type of network known in the art. Each of the PLCs 30 may be connected to a plurality of field devices 50, each field device providing analog data regarding particular industrial process or action. Field devices 50 may be, for example, pressure sensors that measure the pressure in an oil tank and provide the pressure data to the PLC; temperature sensors measuring the temperature of a particular substance, etc. In turn, PLCs 30 may provide instructions to field devices as an automated feedback in response to the data received from the field devices. The system server 20 typically controls overall automated process.
Field devices may provide data to PLCs in a variety of communication formats, such as formats that utilize the frequency-shift-keying technique (FSK). FSK, or frequency-shift keying, is a frequency modulation scheme in which digital information is transmitted through discrete frequency changes of a carrier wave. The simplest FSK is binary FSK (BFSK), which utilizes multiple discrete frequencies to transmit binary (0s and 1s) information. With this scheme, the “1” is called the mark frequency and the “0” is called the space frequency. The Highway Addressable Remote Transducer (HART) protocol is a type of FSK that will be described below in greater detail.
To facilitate data exchange between PLCs and field devices, FSK modems are used in PLC systems. Typically, FSK modems used in such systems incorporate the HART protocol. HART provides digital communication to microprocessor-based (smart) analog process control instruments. A HART modem is a type of FSK modem in which the frequency, bit format or frame, preamble and other physical parameters are specified along with a protocol for messaging between modems. FSK modems in PLCs are incorporated to send information using HART over inputs and outputs used to transmit and receive other analog information. An example would be an output of one PLC, which is transmitting a voltage to another PLC input wherein both PLCs incorporate FSK modems that are HART enabled. In addition to the voltage signal, a data signal can be superimposed on the voltage measurement that consists of serial characters used to denote other important data of merit. Examples of the data of merit include, for example, an ID of a machine or operation, and the like. For convenience, “FSK” and “HART” will be used interchangeably in the present application.
PLC modules can communicate with field devices, such as sensors or actuators, using the HART communications link that includes a physical and potentially multiple protocol layers. Normally, the physical layer of HART is based on using a 1200 Hz or 2200 Hz continuous phase tones to represent “ones” and “zeros,” which is a type of FSK as described above. The tones are transmitted and received over a current or voltage loop, modulating the tone's amplitude, typically around 500 mV.
Typically, FSK modem functionality in PLCs is implemented using off-the-shelf Hart Modems, such as the A5191HRT modem by AMI Semiconductors or the HT2015 by Smar Research or the SYM20C15 by Symbios Logic.
A disadvantage of existing off-the-shelf modems is that they need to be connected to a universal asynchronous receiver/transmitter (UART). In this regard, a UART is a separate piece of computer hardware that translates data between parallel and serial forms in a micro-controller. This may require that at least one additional micro-controller be dedicated to the modem, adding cost and consuming within a PLC, or additional UARTs, also adding to cost and space.
The quality of the output tones generated by existing modems is poor, sometimes barely meeting the HART specification. Moreover, another disadvantage of existing off-the-shelf systems is the inability to add design elements for improving the quality of the FSK modem operation. Design elements that improve FSK modem operation may include, but are not limited to, electronic and software filters, adaptive control algorithms, and the like.
PLC inputs and/or outputs often are required to be electrically isolated from the power circuitry within the PLC system. A traditional PLC communication topology 28 employing a stand-alone HART modem is illustrated in FIG. 2A. The topology comprises a PLC main controller 30 connected to a field device 50 through an analog-to digital converter 68 and HART modem 65. Typically, this implementation requires an isolation barrier 60, for example, a transformer, to be located to isolate the HART modem signal 64 from the HART modem 65 to the PLC 30 as shown in FIG. 2A. This arrangement adds cost, consumes considerable space, and can further degrade HART output tones.
It is often desirable to have multiple FSK channels in a single PLC. In multi-channel modules the performance is low if the modem is shared by the channels. In case of isolated input/output channels each channel is isolated by a transformer if sharing a modem, or requires a modem per channel, adding to cost and board space.
Therefore, it would be beneficial to have a device that would obviate the need for one UART per FSK modem in a multi-channel design, increase the density and lower the cost of FSK channels, improve the FSK modem signal fidelity, and eliminate the need to share a modem in a multi-channel design.